Enhanced light extraction through the use of micro-LED arrays

ABSTRACT

This invention describes new LED structures that provide increased light extraction efficiency. The new LED structures include arrays of electrically interconnected micro-LEDs that have and active layer sandwiched between two oppositely doped layer. The micro-LEDs are formed on a first spreader layer with the bottom layer of the micro-LEDs in contact with the first spreader. A second spreader layer is formed over the micro-LEDs and in contact with their top layer. The first spreader layer is electrically isolated from the second spreader layer. Each of the spreader layers has a contact and when a bias is applied across the contacts, current spreads to the micro-LEDs and they emit light. The efficiency of the new LED is increased by the increased emission surface of the micro-LEDs. Light from each of the micro-LEDs active layer will reach a surface after travelling only a short distance, reducing total internal reflection of the light. Light extraction elements (LEEs) between the micro-LEDs can be included to further enhance light extraction. The new LEDs are fabricated with standard processing techniques making them highly manufacturable at costs similar to standard LEDs.

This application claims the benefit of provisional application number60/168,817 to Thibeault et al., which was filed on Dec. 3, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to light emitting diodes and more particularly tonew structures for enhancing their light extraction.

2. Description of the Related Art

Light emitting diodes (LEDs) are an important class of solid statedevices that convert electric energy to light and commonly comprise anactive layer of semiconductor material sandwiched between two oppositelydoped layers. When a bias is applied across the doped layers, holes andelectrons are injected into the active layer where they recombine togenerate light. The light generated by the active region emits in alldirections and light escapes the device through all exposed surfaces.Packaging of the LED is commonly used to direct the escaping light intoa desired output emission profile.

As semiconductor materials have improved, the efficiency ofsemiconductor devices has also improved. New LEDs are being made frommaterials such as GaN, which provides efficient illumination in theultra-violet to amber spectrum. Many of the new LEDs are more efficientat converting electrical energy to light compared to conventional lightsand they can be more reliable. As LEDs improve, they are expected toreplace conventional lights in many applications such as trafficsignals, outdoor and indoor displays, automobile headlights andtaillights, conventional indoor lighting, etc.

However, the efficiency of conventional LEDs is limited by theirinability to emit all of the light that is generated by their activelayer. When an LED is energized, light emitting from its active layer(in all directions) reaches the emitting surfaces at many differentangles. Typical semiconductor materials have a high index of refraction(n≈2.2-3.8) compared to ambient air (n=1.0) or encapsulating epoxy(n≈1.5). According to Snell's law, light traveling from a region havinga high index of refraction to a region with a low index of refractionthat is within a certain critical angle (relative to the surface normaldirection) will cross to the lower index region. Light that reaches thesurface beyond the critical angle will not cross but will experiencetotal internal reflection (TIR). In the case of an LED, the TIR lightcan continue to be reflected within the LED until it is absorbed, or itcan escape out surfaces other than the emission surface. Because of thisphenomenon, much of the light generated by conventional LEDs does notemit, degrading efficiency.

One method of reducing the percentage of TIR light is to create lightscattering centers in the form of random texturing on the surface.[Shnitzer, et al., “30% External Quantum Efficiency From SurfaceTextured, Thin Film Light Emitting Diodes”, Applied Physics Letters 63,Page 2174-2176 (1993)]. The random texturing is patterned into thesurface by using sub micron diameter polystyrene spheres on the LEDsurface as a mask during reactive ion etching. The textured surface hasfeatures on the order of the wavelength of light that refract andreflect light in a manner not predicted by Snell's Law due to randominterference effects. This approach has been shown to improve emissionefficiency by 9-30%.

One disadvantage of surface texturing is that it can prevent effectivecurrent spreading in LEDs which have poor electrical conductivity forthe textured electrode layer, such as the case of p-type GaN. In smallerdevices or devices with good electrical conductivity, current from the pand n-type layer contacts spreads throughout the respective layers. Withlarger devices or devices made from materials having poor electricalconductivity, the current cannot spread from the contacts throughout thelayer. As a result, part of the active layer does not experience thecurrent and will not emit light. To create uniform current injectionacross the diode area, a spreading layer of conductive material isdeposited on its surface. However, this spreading layer often needs tobe optically transparent so that light can transmit through the layer.When a random surface structure is introduced on the LED surface, aneffectively thin and optically transparent current spreader cannoteasily be deposited.

Another method of increasing light extraction from an LED is to includea periodic patterning in the emitting surface or internal interfaceswhich redirects the light from its internally trapped angle to definedmodes determined by the shape and period of the surface. See U.S. Pat.No. 5,779,924 to Krames et at. This technique is a special case of arandomly textured surface in which the interference effect is no longerrandom and the surface couples light into particular modes ordirections. One disadvantage of this approach is that the structure canbe difficult to manufacture because the shape and pattern of the surfacemust be uniform and very small, on the order of a single wavelength ofthe LED's light. The pattern can also present difficulties in depositingan optically transparent current spreading layer as described above.

An increase in light extraction has also been realized by shaping theLED's emitting surface into a hemisphere with an emitting layer at thecenter. While this structure increases the amount of emitted light, itsfabrication is difficult. U.S. Pat. No. 3,954,534 to Scifres and Burnhamdiscloses a method of forming an array of LEDs with a respectivehemisphere above each of the LEDs. The hemispheres are formed in asubstrate and a diode array grown over them. The diode and lensstructure is then etched away from the substrate. One disadvantage ofthis method is that it is limited to formation of the structures at thesubstrate interface, and the lift off of the structure from thesubstrate results in increased manufacturing costs. Also, eachhemisphere has an emitting layer directly above it, which requiresprecise manufacturing.

U.S. Pat. No. 5,793,062 discloses a structure for enhancing lightextraction from an LED by including optically non-absorbing layers toredirect light away from absorbing regions such as contacts and alsoredirect light toward the LED's surface. One disadvantage of thisstructure is that the non-absorbing layers require the formation ofundercut strait angle layers, which can be difficult to manufacture inmany material systems.

Another way to enhance light extraction is to couple photons intosurface plasmon modes within a thin film metallic layer on the LED'semitting surface, which are emitted back into radiated modes. [Knock etal., ‘Strongly Directional Emission from AlGaAs/GaAs Light EmittingDiodes’ Applied Physics Letter 57, pgs. 2327-2329 (1990)]. Thesestructures rely on the coupling of photons emitted from thesemiconductor into surface plasmons in the metallic layer, which arefurther coupled into photons that are finally extracted. Onedisadvantage of this device is that it is difficult to manufacturebecause the periodic structure is a one-dimensional ruled grating withshallow groove depths (<0.1 μm). Also, the overall external quantumefficiencies are low (1.4-1.5%), likely due to inefficiencies of photonto surface plasmon and surface plasmon-to-ambient photon conversionmechanisms. This structure also presents the same difficulties with acurrent spreading layer, as described above.

Light extraction can also be improved by angling the LED chip's sidesurfaces to create an inverted truncated pyramid. The angled surfacesprovide TIR light trapped in the substrate material with an emittingsurface within the critical angle [Krames et al., ‘High Power TruncatedInverted Pyramid (Al_(x)Ga_(1−x))_(0.5)In_(0.5)P/GaP Light EmittingDiodes Exhibiting >50% External Quantum Efficiency’ Applied PhysicsLetters 75 pgs. 2365-2367 (1999)]. Using this approach external quantumefficiency has been shown to increase from 35% to 50% for the InGaAlPmaterial system. This approach works for devices in which a significantamount of light is trapped in the substrate. For the case of GaN onsapphire, much of the light is trapped in the GaN film so that anglingthe LED chip's side surfaces will not provide the desired enhancement.

Still another approach for enhancing light extraction is photonrecycling [Shnitzer, et al., ‘Ultrahigh spontaneous emission quantumefficiency, 99.7% internally and 72% externally, from AlGaAs/GaAs/AlGaAsdouble heterostructures’, Applied Physics Letters 62, Page 131-133(1993)]. This method relies on LEDs having a high efficiency activelayer that readily converts electrons and holes to light and vice versa.TIR light reflects off the LED's surface and strikes the active layer,where the light is converted back to an electron-hole pair. Because ofthe high efficiency of the active layer, the electron-hole pair almostimmediately reconverts to light that is again emitted in randomdirection. A percentage of this recycled light strikes one of the LEDsemitting surfaces within the critical angle and escapes. Light that isreflected back to the active layer goes through the same process again.However, this approach can only be used in LEDs made from materials thathave extremely low optical loss and cannot be used in LEDs having anabsorbing current spreading layer on the surface.

SUMMARY OF THE INVENTION

The present invention provides a class of new LEDs having interconnectedarrays of micro-LEDs to provide improved light extraction. Micro-LEDshave a smaller active area, in the range of 1 to 2500 square microns,but the size is not critical to the invention. An array of micro-LEDs isany distribution of electrically interconnected micro-LEDs. The arraysprovide a large surface area for light to escape each of the micro-LEDs,thereby increasing the usable light from the LED. The new LED can havemany different geometries and because it is formed by standardsemiconductor process techniques, it is highly manufacturable.

The new LED includes a conductive first spreader layer with micro-LEDsdisposed on one of its surfaces. Each micro-LED has a p-type layer, ann-type layer and an active layer sandwiched between the p- and n-typelayers. Either the p- or n-type layer is a top layer and the other isthe bottom layer. Current applied to the first spreader layer spreadsinto each micro-LED's bottom layer. A second spreader layer is includedover the micro-LEDs and current from said second spreader spread intothe top layer. When a bias is applied across the first and secondspreader layers the micro-LEDs emit light.

One embodiment of the second spreader is a conductive interconnectedgrid-like structure having conductive paths over the micro-LEDs, incontact with the top layer of the micro-LEDs. An insulator layer isincluded over the array with the grid on the insulator layer, therebyelectrically isolating the first spreader layer from the grid.

Alternatively, flip-chip bonding can be used to interconnect themicro-LEDs. Using this method, an unconnected micro-LED array is firstformed and then bonded to an electrically conductive material to providethe array interconnection. In a third embodiment, the grid passes overthe micro-LEDs and the p-type, active, and n-type material is under theconductive paths of the grid between the micro-LEDs to electricallyisolate the grid from the first spreader layer. This grid-like structurecan be designed so that emitted light interacts with a sidewall aftertraveling a small distance.

The new LED can have LEEs disposed between the micro-LEDs or formed onthe side surfaces of the micro-LEDs, to further enhance lightextraction. The LEEs act to redirect or focus light that would otherwisebe trapped or absorbed through TIR in a standard LED structure. Theirshapes may be curved (convex or concave) or piecewise linear with theshape of the structure affecting the light extraction and final outputdirection of light. LEEs that are placed between the micro-LEDs interactwith light escaping from the sides of the micro-LEDs. This interactionhelps prevent the light from reflecting back into the LED to beabsorbed, thereby increasing the useful light out of the LED.

These and other further features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription, taken together with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a the new LED with a micro-LED array having aninterconnecting current spreading grid on an electrically insulatinglayer;

FIG. 2 is a sectional view of the new LED shown in FIG. 1 taken alongsection lines 2—2;

FIG. 3 is a sectional view of another embodiment of the new LED with itsmicro-LED array bonded to a submount using flip-chip mounting;

FIG. 4 is a plan view of a third embodiment of the new LED conductiveinterconnect current spreading grid and semiconductor material below thegrid paths;

FIG. 5 is a sectional view of the LED shown in FIG.4, taken alongsection lines 4—4;

FIG. 6 is a plan view of an alternative interconnecting currentspreading grid;

FIG. 7 is a plan view of another alternative of an interconnectingcurrent spreading grid;

FIG. 8 shows sectional views of the basic shapes of LEEs that can beintegrated within the micro-LEDs;

FIG. 9 is a sectional view of the new LED with different LEEs formedbetween the micro-LEDs;

FIG. 10 is a sectional view of the new LED with LEEs in the form ofrandomly textured surfaces;

FIG. 11 is a sectional view of the micro-LED array in FIG. 10, having acurrent blocking layer directly underneath the current spreading grid;

FIG. 12 is a sectional view of the new LED with LEEs integrated on themicro-LED side surfaces;

FIG. 13 is a sectional view the new LED with curved surface LEEsintegrated on the sides of the micro-LEDs;

FIG. 14 is sectional view of the new LED with curved surface LEEsintegrated on the sides of and between the micro-LEDs; and

FIG. 15 is a sectional view the new LED of FIG. 4 with curved LEEs.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 show one embodiment of the new LED 10 constructed inaccordance with the present invention. It includes an array ofmicro-LEDs 12 with each micro-LED 12 being isolated and having its ownactive layer of semiconductor material 14 sandwiched between twooppositely doped layers 16 and 18. In the preferred micro-LED, the toplayer 16 is p-type and the bottom layer 18 is n-type, although oppositedoping in the layers 16, 18 will also work.

The new LED also includes a first spreading layer 20 that spreadscurrent from the n-contact pad 22 to each micro-LED's bottom layer 18.The contact pad 22 is referred to as the n-contact pad because in thepreferred embodiment the bottom layer 18 is n-type. An insulating layer23 is deposited over the micro-LED array, covering each micro-LED andthe surface of the first spreader in the gaps between the micro-LEDs. Asecond spreading layer, preferably in the form of an interconnectedcurrent spreading grid 24, is deposited on the insulating layer with thegrid's conductive paths passing over the micro-LEDs. A p-contact 26 isdeposited on the grid 24 and current from the contact spreads throughthe grid to top layer 16 each micro-LED 12. The contact 26 is referredto as the p-contact because in the preferred embodiment the top layer 16is p-type.

A hole is formed through the insulating layer on top of each micro-LEDand a micro-LED contact 29 is included in each insulating layer hole toprovide contact between the grid 24 and the micro-LED's top layer 16.The micro-LEDs (except for the holes) and the surface of the firstspreading layer are electrically isolated from the current spreadinggrid by the insulating layer 23. The entire structure is formed on asubstrate 28 and the micro-LEDs form an array that emits light when abias is applied across the contacts 22 and 26. In another embodiment, atransparent conductive sheet is used as the second spreader in place ofthe grid 24.

The new LED 10 has enhanced light emission because of the increasedemission surface area provided by the isolated micro-LEDs. Lightgenerated from each micro-LED's active layer interacts with the edge ofthe micro-LED after only a very short distance. If the light is withinthe critical angle, it escapes from the micro-LED and contributes to theLED's light emission. The new LED is especially useful for LEDstructures in which a portion of emitted light cannot be transmitted tothe substrate due to total internal reflection (TIR) at the currentspreader-substrate interface. This is the situation for GaN-based LEDson sapphire, AlN, or MgO substrates.

The new LED 10 is manufactured by first depositing the first spreaderlayer 20 on the substrate 28. An epitaxially grown LED structure with ann-type, p-type and an active layer, is then formed on the first spreaderlayer 20. The micro-LEDs are formed from the LED structure by etchingaway portions of the structure using semiconductor etching techniquessuch as wet chemical etching, RIE, Ion Milling, or any other techniqueused for removing semiconductor material.

Each remaining micro-LED forms an independent and electrically isolateddevice having an active layer surrounded by oppositely doped layers 16and 18. The shape and position of the micro-LEDs 12 can be varied withthe preferred shape of each micro-LED being cylindrical. When viewedfrom above, each micro-LED appears circular with a diameter of between 1and 50 microns. The micro-LEDs are preferably formed in a close packedpattern to maximize the usable micro-LED space. The separation betweenadjacent micro-LEDs is preferably in the range of 1 to 50 μm, althoughthe separation is not critical to this invention. The insulator layer 23is deposited over the entire structure by various methods such asevaporation, CVD or sputtering. Openings are then etched in theinsulator layer 23 above each micro-LED 12. The micro-LED contacts andthe electrically conductive grid are then deposited by standarddeposition techniques.

The first spreader layer 20 may be either a conductive layer depositedon the substrate or the substrate itself, if it is conductive. Preferredconductive substrates for GaN-based LEDs include GaN or Silicon Carbide(SiC). SiC has a much closer crystal lattice match to Group III nitridessuch as GaN and results in Group III nitride films of high quality.Silicon carbide also has a very high thermal conductivity so that thetotal output power of Group III nitride devices on silicon carbide isnot limited by the thermal dissipation of the substrate (as is the casewith some devices formed on sapphire). SiC substrates are available fromCree Research, Inc., of Durham, N.C. and methods for producing them areset forth in the scientific literature as well as in a U.S. Pat. Nos.Re. 34,861; 4,946,547; and 5,200,022.

If the substrate is the current spreading layer, the bottom contact canbe deposited by metalization on any of the substrate's exposed surfaces.The preferred LED has micro-LEDs 12 that are AlGaInN based with a p-typesurface as its top layer 16. The substrate is sapphire, the firstspreader is n-type AlGaInN (or an alloy thereof), and the contactmetalization is Al/Ni/Au, Al/Ti/Au, or Al/Pt/Au. The insulating layer 23can be made of many materials such as, but not limited to, SiN, SiO2, orAlN.

The grid 24 can be any electrically conductive material includingmetals, semi-metals, and semiconductors. It can be made of Al, Ag,Al/Au, Ag/Au, Ti/Pt/Au, Al/Ti/Au, Al/Ni/Au, Al/Pt/Au or combinationsthereof. Alternatively, the grid can be made of a thin semi-transparentmetal such as Pd, Pt, Pd/Au, Pt/Au, Ni/Au, NiO/Au or any alloy thereof.The grid 24 can be deposited on the new LED by many conventional methodswith the preferred methods being evaporation or sputtering. In thepreferred embodiment, the paths of the current spreading grid 24 arebetween 1 and 10 μm wide. The micro-LED contacts 29 can be made of Pt,Pt/Au, Pd, Pd/Au, Ni/Au, NiO, or NiO/Au. The p-contact 26 can bedeposited on the interconnected grid 24 in various locations to allowcurrent from the contact to spread throughout the grid.

FIG. 3 shows a second embodiment of the LED 30 constructed in accordancewith the present invention, utilizing flip-chip mounting. As above,micro-LEDs 32 are formed in an array by etching away semiconductormaterial of a full LED structure. Each micro-LED 32 has an active layersurrounded by two oppositely doped layers. The micro-LED arrangement andsize is similar to the embodiment described above. However, in thisembodiment each of the micro-LEDs has angled side surfaces and their toplayer is narrower than its bottom layer. Like above, the micro-LED arrayis formed on a first spreader layer 34 that is formed on a substrate 36.An insulating layer 38 covers the micro-LEDs and the surface of thefirst spreader between adjacent micro-LEDS. On each micro-LED 32, a holeis included in the insulating layer for a top contact 40 A secondspreader layer 42 coats the entire micro-LED array to interconnect thetop contacts 40.

The surface of the second spreader 42 opposite the micro-LEDs is bondedto a reflective metallic layer 48 on a submount 46 by a bonding media. Ap-contact 44 is included on the metallic layer 48 and current applied tothe second contact spreads throughout the second spreader, to the topcontacts 40 and to the top layer of the micro-LEDs 32. There is a breakin the metallic layer 48 and the n-contact 50 is formed on the portionof the metallic layer 48 that is electrically isolated from the portionhaving the p-contact. The finger 49 is bonded between the submount andthe first spreader and conducts current from the contact 50 through themetallic layer 48, through the finger and to the first spreader 34. Thecurrent then spreads throughout the first spreader and to the bottomlayer of the micro-LEDs.

In this flip-chip embodiment, light from the LED 50 is primarily emittedthrough the substrate 36. The second spreader 42 can be opticallyreflective so that light emitted from the micro-LEDs 32 in the directionof the second spreader 42 is reflected towards the LED's substrate 36.Al or Ag is preferably used as the second spreader and each micro-LED 32is AlGaInN based with a p-type top layer. Each top contact 40 ispreferably Pt, Pt/Au, Pd, Pd/Au, Ni/Au, NiO, or NiO/Au.

This embodiment provides increased sidewall interaction with the emittedlight as a result of the isolated micro-LEDs. The portion of the secondspreader 42 that is disposed between the micro-LEDs functions as LEEs byreflecting light from the micro-LEDs toward the substrate. Thisconfiguration also provides improved heat transfer out of the new LEDchip through the submount.

FIGS. 4 and 5 show another embodiment of the new LED 51 that does nothave an insulator layer to isolate the first spreader from the second.Instead, micro-LEDs 52 are connected to adjacent micro-LEDs byconductive paths 53 of an interconnected grid 54, wherein the paths havesemiconductor material below them. Each opening 55 in the grid 54 is anarea where semiconductor material was etched from the LED structure whenmanufacturing the LED 50. Portions of the structure remain under thegrid 54 as micro-LEDs 52 and as semiconductor material under the gridpaths 53 between the micro-LEDS. The micro-LEDs and the material underthe paths comprise an active layer surrounded by two oppositely dopedlayers, with the entire structure formed on a first spreader layer 56and a substrate 58.

A first contact 60 is deposited on the first spreader to apply currentto the bottom layer of the micro-LEDs and a second contact 62 isincluded on the current spreading grid to spread current to the toplayer of the micro-LEDs. When a bias is applied across the contacts 60and 62, current is applied to the micro-LEDs and the semiconductormaterial under the conductive paths, all of which emit light. Lightescapes from the side surfaces of the micro-LEDs material under thepaths, avoiding total internal reflection. This technique is thereforegenerally applicable to any LED structure on any substrate and isimplemented with standard processing techniques.

The LED 51 is manufactured by first depositing the first spreader layer56 on the substrate 58, and then forming a continuous LED structure thatcovers the current spreading layer 56. The grid 54 is deposited on theLED structure and portions of the LED structure that are visible in thegrid openings are etched away by various methods such as wet chemicaletching, Reactive Ion Etching (RIE), Ion Milling, or any other techniqueused for removing semiconductor material. Portions of the LED structureare also etched to provide an area for contact pads 60, and contact pads60 and 62 are deposited. The grid 54 can be made of any electricallyconductive material including but not limited to metals, semi-metals,and semiconductors or combinations thereof. The preferred micro-LEDs areGaN-based with each micro-LED's top layer 55 being a p-type AlGaInN orany alloy thereof, and the grid 54 is preferably made of a thin metalsuch as Ni, Pd, Au, Pt or any combination thereof.

The dashed line in FIG. 4 illustrates one of the micro-LEDs and the areaaround the micro-LED where LEEs can be included to further enhance lightextraction as more fully described below.

FIGS. 6 and 7 show two additional embodiments 70 an 80 of the new LEDwith different micro-LED and grid patterns 72 and 82, although manydifferent patterns can be used. Each embodiment has a respective bottomspreading contact 73 and 83. In FIG. 6, the micro-LEDs 74 areinterconnected crosses with current spreading to each of the micro-LEDsthrough the various paths. Each path has semiconductor material below itto isolate it from the first spreader layer. The grid 72 provides asquare array pattern of openings for light interaction.

The grid 54 has an advantage over grid 72. In LED 70, TIR light canreflect down one of the grids numerous conductive paths and reflectwithin the LED without interacting with a surface of one of themicro-LEDs. Optical loss present in the grid or underlying layers willcause some of this TIR light to be absorbed before it can escape out thefinal edge of the new LED. The grid 54 reduces this problem becauselight emitting from the micro-LEDs will reach a side surface aftertravelling only a short distance (at most two micro-LED lengths),thereby increasing the light out of the device.

In FIG. 7 the micro-LEDs are randomly shaped and have randominterconnecting paths. Again, the paths have semiconductor materialbelow them. The random pattern reduces the number of paths for the TIRto travel before it will encounter one of the micro-LEDs in one of thegrid openings. Like above, the dashed lines around the micro-LEDs inFIGS. 6 and 7 illustrate the micro-LED 76 and 86 with LEEs around theirperimeters, as more fully described below.

Opening sizes and distances between openings are preferably between 1and 30 μm, but may be larger or smaller. The pattern of the openings maybe aperiodic or periodic since the nature of the light interaction withthe micro-LED edges does not require either condition. In the preferredembodiment on a p-type AlGaInN layer, the grid openings are between 1micron and 20 μm and the micro-LEDs have a width between 1 μm and 30 μm.

All of the previous three embodiments can be integrated with LEEsbetween micro-LEDs to further increase light emission. The LEEs caneither be formed on the side surfaces of the micro-LEDs or on thesurface of the first spreader layer or the conductive substrate in thoseembodiments having no first spreader layer.

FIG. 8 shows several alternative shapes of LEEs that are included asembodiments in this invention, although other shapes can be used and thescope of this invention is not limited to the shapes shown. LEEs 82, 84,86 have curved surfaces while the LEEs 88, 90, 92, 94 have piecewiselinear surfaces. Alternatively, the LEE may be a randomly roughenedlayer that acts as a light disperser.

The LEEs can be formed by standard etching techniques such as wetchemical etching, RIE, or ion milling. In the preferred embodiment, theLEEs are formed by using a commercially available polymer (such as a UVor e-beam sensitive photoresist) as an ablative etch mask. This polymeris first deposited and patterned with square-like edges. The polymer isheated to a temperature and reflows like glass to give a gradual linearor curved shape to the edges of the polymer. The polymer thickness,pattern shape, heating temperature and heating time will determine theedge shape. The pattern is transferred to the AlGaInN based micro-LEDswith RIE. Linear and curved LEEs are easily fabricated by this methodand piecewise linear LEEs can be easily formed by using multipleablative masks.

A second technique for forming LEEs is to use a negative polarityUV-exposable photoresist. First, the photoresist is exposed for aparticular exposure time and is treated to produce a negative polarity.The photoresist is then developed to produce an undercut curved orlinear shape in its profile. This pattern can then be transferred to thesemiconductor material through a dry etching technique. For bothembodiments, the dry etching conditions will also impact the final shapeof the lens in the semiconductor material.

FIGS. 9-15 show embodiments of the new LED with LEEs integrated withinthe micro-LED array in a variety of ways to enhance light extraction.These embodiments represent a few of the possible ways that the LEEs canbe used in accordance with this invention, and the scope of thisinvention is not limited to the described embodiments.

FIG. 9 shows a new LED 100 that is similar to the LED 50 in FIGS. 4 and5, but has LEEs 101, 102, 103 between the micro-LEDs 104. The LEEs 101,102, 103 allow light that is directed through a micro-LED's side surfaceto reflect off the LEEs and be re-directed away from the substrate intoa package. Light rays that reflect off of the interface between thesubstrate 108 and first spreader layer 106 through TIR can also interactwith the LEEs 101, 102, 103 to escape into the package, providing ahigher light output. The LEEs depicted in FIG. 8 can be either depositedonto or processed into the new LED. As described above, the depth of theLEEs can also be varied with the preferred depth in the range of 0.5 μmto 10 μm.

FIG. 10 shows a new LED 110 similar to the LED 100 in FIG. 9, but havingrandomly roughened dispersion LEEs 112 between the micro-LEDs 113. Thelight interaction with the roughened layer allows TIR light to reach thesurface within its critical angle and escape before being absorbed. Inthe preferred embodiment, the roughened surface is formed by usingpolystyrene or silica microspheres as an etch mask to transfermicro-scale roughness into the semiconductor. The depth and width of therandom roughness may be less than 20 nm to more that 500 nm, with thepreferred size being on the order of the wavelength of light generatedby the LED.

FIG. 11 shows a new LED 120 that is similar to LED 110 in FIG. 10, butincludes a current blocking layer within the micro-LED. The blockinglayer 122 directs current flow underneath the dispersive LEE 124,increasing the chance for light to interact with the LEE and escape.

As an alternative to forming the LEEs between the micro-LEDs, the LEEscan be formed directly on the micro-LED side surfaces. FIG. 12 shows anew LED 130 that is similar to the LEDs in FIGS. 9, 10 and 11, buthaving various LEEs 131-133 formed directly on each micro-LED's sidesurfaces. The LEEs can be formed using the same methods as describedabove. Light that travels towards the micro-LED side surfaces isredirected in directions that cause light to escape out one of thesurfaces of the substrate 134, through the first spreader layer 135, orthrough the micro-LEDs 132. Light that is reflected back from thesubstrate 134 also has an increased chance of escape due to the LEEs onthe micro-LED edges.

FIG. 13 shows a new LED 140 where curved LEEs 142 are formed on the sidesurfaces micro-LEDs 144. The curved LEEs 142 provide the additionaladvantage of focusing the LED light into a more well defined direction.The depth and width of the LEEs 142 can be varied with the preferreddepth of any one LEE being 0.1 μm to 50 μum.

Two additional embodiments are shown in FIGS. 14 and 15. FIG. 14 shows anew LED 150 with a combination of curved LEEs 152 on the side surface ifthe micro-LEDs 154 and full curved LEEs 156 between the micro-LEDs 154.The LEEs work together to enhance light extraction by refracting andreflecting light out of the LED package

FIG. 15 shows new LED 160 with curved LEEs 162 on the side surfaces ofthe micro-LEDs 164, using flip-chip embodiment mounting similar to theembodiment shown in FIG. 3. The second spreader 164 is reflective andthe substrate 166 is the primary emitting surface. The LEEs 162 and theportions of the second spreader 164 work together to enhance lightextraction by refracting and reflecting light out of the LED packagethrough the substrate.

Although the present invention has been described in considerable detailwith reference to certain preferred configurations thereof, otherversions are possible. For instance, the bottom layers of the micro-LEDsin the array can be in contact. The light extraction structures can alsobe used in many different combinations and can be many different shapesand sizes. Also, the LED structure described above can have more thanone active layer sandwiched between oppositely doped layers. Therefore,the spirit and scope of the appended claims should not be limited totheir preferred embodiments described above.

We claim:
 1. A light emitting diode (LED) with enhanced lightextraction, comprising: a conductive first spreader layer; a pluralityof micro light emitting diodes (micro-LEDs) separately disposed on asurface of said first spreader layer, each of said micro-LEDscomprising: a p-type layer; an n-type layer; an active layer sandwichedbetween said p-type and n-type layers wherein either said p-type orn-type layer is a top layer and the other said layer is the bottomlayer, current from said first spreader layer spreading into said bottomlayer; a second spreader layer over said micro-LEDs, current from saidsecond spreader layer spreading into said top layer, a bias appliedacross said first and second spreader layers causing said micro-LEDs toemit light.
 2. The LED of claim 1, wherein said first spreader layer isa conductive substrate.
 3. The LED of claim 1, further comprising asubstrate adjacent to the surface of said first spreader layer oppositesaid micro-LEDs.
 4. The LED of claim 3, wherein said substrate isinsulating and said first spreader layer is an epitaxially depositedsemiconductor material.
 5. The LED of claim 1, further comprising aninsulating layer covering said micro-LEDs and the surface of said firstspreader layer between said micro-LEDs, said insulating layer disposedbetween said second spreader and said micro-LEDs.
 6. The LED of claim 5,wherein said insulating layer has holes over each of said micro-LEDs andsaid second spreader making contact with each said micro-LED throughsaid holes.
 7. The LED of claim 6, wherein said second spreading layeris a sheet of transparent conducting material.
 8. The LED of claim 6,wherein said second spreader layer is an interconnected currentspreading grid having a plurality of interconnected conductive paths,each of said micro-LEDs having one or more conductive paths over it andmaking contact with said top layer through said holes.
 9. The LED ofclaim 6, wherein said second spreader is an electrically conductivematerial.
 10. The LED of claim 1, further comprising light extractionelements (LEEs) integrated with said micro-LEDs to interact with lightescaping from said micro-LEDs to further enhance light extraction fromsaid LED.
 11. The LED of claim 10, wherein said LEEs are disposedbetween said micro-LEDs.
 12. The LED of claim 10, wherein said LEES aredisposed on the surface of said first spreader layer, between saidmicro-LEDs.
 13. The LED of claim 10, wherein said LEEs are integrated onthe side surfaces of said micro-LEDs.
 14. The LED of claim 10, whereinsaid LEEs are integrated on the sides surfaces of said micro-LEDs andare disposed between said micro-LEDs.
 15. The LED of claim 10, whereinsaid LEEs have curved surfaces.
 16. The LED of claim 10, wherein saidLEEs have linear or piecewise linear surfaces.
 17. The LED of claim 10,wherein said LEEs are randomly roughened surfaces.
 18. The LED of claim1, further comprising respective electrical contacts disposed on saidfirst and second spreader layers, a bias applied across said contactscausing said active layer to emit light.
 19. The LED of claim 6, whereinsaid second spreader layer is a reflective metal layer that is depositedover said micro-LEDs, said LED further comprising substrate adjacent tothe surface of said first spreader opposite said micro-LEDs and asubmount layer affixed to said metal layer, said substrate of said LEDbecoming the primary light emission surface.
 20. The LED of claim 19,further comprising a conductive finger between said submount and saidfirst spreader layer, a first contact on said submount connected to saidconductive finger, and a second contact on said submount connected tosaid metal layer, said micro-LEDs emitting light when a bias is appliedacross said contacts.
 21. The LED of claim 1, wherein said bottom layersof said micro-LEDs are connected and said active and top layers aredisconnected.
 22. The LED of claim 1, wherein said second spreader layeris an interconnected current spreading grid with conductive pathsbetween said micro-LEDs, said LED further comprising semiconductormaterial under said conductive paths between micro-LEDs, saidsemiconductor material electrically isolating said first spreader layerfrom said conductive paths.
 23. The LED of claim 22, wherein saidsemiconductor material comprises an active layer sandwiched between twooppositely doped layers.
 24. A light emitting diode (LED), comprising: afirst spreader layer; an array of light emitting elements disposed onsaid first spreader layer; a second spreader layer disposed over saidarray of emitting elements, said first spreader layer electricallyisolated from said second spreader layer; and first and second contactson said first and second spreader layer respectively, a bias appliedacross said contacts causing said array of emitting elements to emitlight.
 25. The LED of claim 24, wherein said emitting elements aremicro-LEDs, each having an active layer sandwiched between twooppositely doped layers.
 26. The LED of claim 24, wherein oppositelydoped layers are p and n-type layers, wherein either said p-type orn-type layer is a bottom layer adjacent to said first spreader layer andthe other said layer is the top layer adjacent to said second spreader,current from said first spreader flowing into said bottom layer andcurrent from said second spreader flowing into said top layer.
 27. TheLED of claim 24, further comprising a substrate adjacent to the surfaceof said first spreader layer opposite said emitting elements.
 28. TheLED of claim 24, wherein said second spreader layer is an interconnectedcurrent spreading grid covering said emitting elements and having aplurality of interconnected conductive paths between said emittingelements.
 29. The LED of claim 24, wherein said second spreading layeris a sheet of transparent conducting material.
 30. The LED of claim 28,further comprising an insulating layer covering said emitting elementand the surface of said first spreader layer between said emittingelements, said second layer disposed on said insulating layer, saidinsulating layer electrically isolating said first spreader layer fromsaid second spreader layer.
 31. The LED of claim 30, wherein saidinsulating layer has holes over each of said emitting elements and saidsecond spreader making contact with each said emitting element throughsaid holes.
 32. The LED of claim 31, each of said emitting elementshaving one or more conductive paths over it and making contact with saidemitting elements through said holes.
 33. The LED of claim 28, furthercomprising semiconductor material under said conductive paths betweenemitting elements said semiconductor material electrically isolatingsaid first spreader layer from said conductive paths.
 34. The LED ofclaim 33, wherein said semiconductor material comprises an active layersurrounded by two oppositely doped layers.
 35. The LED of claim 24,further comprising light extraction elements between said emittingelements to redirect light emitting from said emitting elements.
 36. TheLED of claim 24, further comprising light extraction elements on theside surfaces of said emitting elements to redirect light emitting fromsaid emitting elements.
 37. A light emitting diode (LED), comprising: afirst spreader layer; an array of micro-LEDs disposed on said firstspreader layer, a current applied to said first spreader layer spreadingto said micro-LEDs; an interconnected current spreading grid disposedover said micro-LEDs, said grid having conductive paths between saidmicro-LEDs, a current applied to said grid spreading to said micro-LEDS.semiconductor material under said conductive paths between micro-LEDs,said semiconductor material electrically isolating said first spreaderlayer from said conductive paths. first and second contacts on saidfirst and second spreader layer respectively, a bias applied across saidcontacts causing said array of emitting elements to emit light.
 38. TheLED of claim 37, wherein said semiconductor material comprises an activelayer sandwiched between two oppositely doped layers.